A significant effort has recently been directed to the task of maximizing the overall performance characteristics of discharge lamps. Such an effort has been notable in the area of electrodeless high intensity discharge (HID) lamps which operate using a high pressure gas fill and an inductively coupled high frequency RF current to create the arc discharge. An example of such an electrodeless HID lamp can be found in U.S. Pat. No. 4,810,938 issued to P. D. Johnson et al on Mar. 7, 1989 and assigned to the same assignee as the present invention, such patent being hereby incorporated by reference. In this patent, an electrodeless HID lamp is inductively driven by a high frequency RF current source which produces an arc discharge within an arc tube containing a gas fill comprising a combination of sodium halide and cerium halide along with xenon gas in proper weight proportions to generate a white color lamp emission exhibiting improved efficacy and color rendering properties.
In order to produce the arc discharge within the arc tube, an electrodeless HID lamp must either capacitively or inductively couple a high frequency RF current to the gas fill of the arc tube. Such a high frequency RF current can be produced by a ballast circuit such as described in U.S. Pat. No. 4,812,702 issued to J. M. Anderson on Mar. 14, 1989 and assigned to the same assignee as the present invention, U.S. Pat. No. 4,812,702 being hereby incorporated by reference. In this patent, an excitation coil is disposed in surrounding relation to the arc tube so that inductively coupled high frequency RF current flowing in such excitation coil results in a time-varying magnetic field which in turn, produces an electric field within the arc tube that substantially closes upon itself. As a result of this solenoidal electric field produced within the arc tube, a toroidally shaped arc discharge is produced in the fill. The excitation coil of this patent is formed having a plurality of turns arranged upon the surface of a torus in a generally V-shaped cross-sectional manner. A tapped reactance impedance matching arrangement is also coupled to the excitation coil. Though this ballast arrangement for an electrodeless HID lamp has proven effective for producing the toroidally shaped discharge within the arc tube, the construction of the excitation coil is costly to implement on a mass production basis and moreover, such V-shaped multiple turn arrangement has the further disadvantage that a substantial portion of the light output from the arc discharge is blocked by the coil structure.
One approach to alleviating the problem of a costly and complex coil construction which can block the light output of the arc discharge is illustrated in U.S. Pat. No. 5,039,903 which issued to G. A. Farrall on Aug. 13, 1991 and is assigned to the same assignee as the present invention, this patent hereby being incorporated by reference. The coil configuration of the Farrall patent minimizes the problem of the excitation coil blocking light output by providing a reduced profile excitation coil. The excitation coil of the Farrall patent comprises one or more coil turns connected in series. The shape of each turn is generally formed by rotating a bilaterally symmetric trapezoid about a coil center line situated in the same plane as the trapezoid, but which line does not intersect the trapezoid, and providing a crossover means for achieving the series connection of the turns. Though this approach has provided an advantage over previous designs in terms of avoiding light blockage by the excitation coil, this approach can be costly to implement on a commercial basis because of the requirement that the coil use a high conductivity copper with an associated investment casting process that can also be costly. In addition, such approach requires the use of a number of braze joints to series connect the two individual turns together and also to connect the other ends of the coil turns to capacitive elements that are necessary to develop the resonant frequency for driving the arc discharge. Of further consideration in terms of applying this approach for commercial mass production purposes is the fact that the thermal management properties of this device require a significant cost and manufacturing time expenditure as well. One example of a thermal management technique discussed in this patent is the use of a liquid cooling channel formed within the coil turns, such an approach having an obvious cost disadvantage. Therefore, it would be advantageous to provide an L-C circuit arrangement for an electrodeless high intensity discharge lamp which would exhibit low energy loss properties and would be operable by means of a high frequency RF current and further, where such L-C circuit were constructed in a manner that would lend itself to mass production manufacturing techniques and would utilize materials and thermal management techniques that would not unduly increase the overall cost of the lamp end product.